nl80211: Support vendor scan together with normal scan

nl80211: Support vendor scan together with normal scan

Allow wpa_supplicant to use vendor scan (if supported by the driver)
together with the normal nl80211 scan and handling external scan events.
Since this results in possibility of concurrent scan operations, some of
the operations related to scan results need to check more carefully when
an event is relevant for a specific interface.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>